1. Field of the Invention
The present invention relates to a method of producing sintered silicon nitrides and, more particularly, to a method of producing a high-strength, sintered silicon nitride suitable for use in applications such as automotive engine parts and bearings.
2. Description of the Prior Art
Sintered silicon nitrides have high strength in both room and high temperature conditions, and other excellent properties, such as heat resistance, heat impact resistance, and corrosion resistance. For this reason, large expectations are entertained with sintered silicon nitrides for use in applications such as automotive engine parts and gas turbine components, and such material has already been in practical use in some applications, such as turbo chargers. Further, sintered silicon nitrides are lighter in weight and more resistant to wear than metallic materials, and because of this fact they are receiving attention for their use as sliding wear resistant materials. Actually, entry of sintered silicon nitrides into application areas including automotive parts, such as piston rings and valves, and machine parts, such as bearings, is under way.
Primarily, silicon nitride (Si.sub.3 N.sub.4) is difficult to sinter, which fact makes it impracticable to sinter the material with itself alone. Conventionally, therefore, sintering aids such as Yttrium oxide (Y.sub.2 O.sub.3), aluminum oxide (Al.sub.2 O.sub.3), magnesium oxide (MgO), and spinel (MgAl.sub.2 O.sub.4), are added in sintering the silicon nitride. In the course of sintering, a liquid phase is formed between the Si.sub.3 N.sub.4 powder and the sintering aids, and Si.sub.3 N.sub.4 powder is dissolved in the liquid phase to precipitate as fine crystalline phase. It has been considered that a microstructure of a sintered material is formed through such a dissolution/precipitation mechanism. Therefore, the microstructure of a sintered material varies and, at same time, characteristics of the sintered material also vary according to the ingredients, composition, and amount of addition of sintering aids.
As such, attention has hitherto been directed to the selection of sintering aids in order to improve mechanical properties of sintered silicon nitrides. One typical method for improving the strength characteristics of sintered silicon nitrides through selection of sintering aids is disclosed in, for example, Japanese Patent Laid-Open Publication SHO 59-131579, wherein a powder mixture including 70 to 94% by weight of silicon nitride, and not less than 2% by weight of yttrium oxide and not less than 2% by weight of spinel, the last two mentioned items totaling 6 to 30% by weight, is molded, the molded piece being then sintered at temperatures of 1650 to 1800.degree. C. in a non-oxidizing atmosphere. In this publication it is stated that sintered materials having a bending strength of 794 to 931MPa (81 to 95 kgf/mm.sup.2) were obtained according to the method.
In Japanese Patent Laid-Open Publication HEI 4-77363 there is described a method wherein 91.5 to 96.99% by weight of silicon nitride powder, 3 to 8% by weight of rare earth element oxide powder, and 0.01 to 0.5% by weight of spinel powder are mixed together, and the mixture is molded into shape, the molded piece being then sintered at atmospheric pressure in a non-oxidizing atmosphere and at temperatures of 1700.degree. to 1850.degree. C. The publication states that a sintered material having good strength characteristics at high temperatures can be obtained according to the method. The four-point bending strength disclosed therein is within the range of 800 to 1035 MPa (81.6 to 105.6 kgf/mm.sup.2) at room temperature, and within the range of 600 to 765 MPa (61.2 to 78.0 kgf/mm.sup.2) at 1300.degree. C.
According to the prior art practice represented by the foregoing two methods, however, the 4-point bending strength of sintered silicon nitrides is of the order of 980 MPa (100 kgf/mm.sup.2) at best. Therefore, when considering applications, such as automotive valve trains and high-speed bearings, which are likely subject to extra large stress and/or impact at sliding with the other parts, such a level of strength characteristics is still far from being said to be sufficient.